1. Field of the Invention
The invention relates to an apparatus for sorting wood chips and more particularly to an apparatus for sorting wood chips according to their surface to weight ratio or thickness.
2. Description of the Related Art
The invention relates to the sorting of wood chips having a variety of sizes and similar, substantially hexahedral shapes having a length and a width, as well as a thickness substantially less than the latter, and possibly fines particles, by means of an impulse action such that the excess thickness chips or the fines particles, or both the excess thickness chips and the fines particles are separated to form a separate fraction. The miscellaneous chips and the fines particles being sorted from each other by means of one or more separating walls arranged to comply with various trajectories imparted to the wood chips by means of the impulse action. An apparatus of this type comprising a feeder for supplying the wood chips onto a sloping support surface and for sliding the same along said support surface over a slit extending thereacross, said slit being adapted to provide an impulse action therethrough by means of a continuous gas flow on the chips or the fines particles as they pass over the slit.
An apparatus of the above type is known from the Applicant's Finnish utility model No. 3899. Such an apparatus is intended for the separation of various fractions from a major mass flow of wood chips (hundreds of bulk cubic meters an hour in volume), as determined by chip thickness. At the same time, it also enables the separation of higher density stuff, such as scrap, as well as irregularly shaped harmful particles, the same way as fines particles. This type of separation is needed in pulp production for processing feed stock.
In this type of apparatus, a plane defined by the width and length of a chip is subjected to a dynamic gas pressure (in practice atmospheric pressure) for a given short period for creating a constant effect per unit area. This results in an impulse action, which brings about a change of velocity for the chip as follows: force=pressure×area or F=pA, impulse=force×application time or I=Ft, and change of velocity=impulse/mass or dv=I/m dv=pAt/m. Since the mass of a chip is dependent on its volume, which is dependent on thickness and area and, on the other hand, the impulse is dependent on area, the entity will be independent of the width and length of a chip, whereby the change of velocity is dependent on a thickness dimension as the material density is constant, such that the ratio of changes in velocity is inversely proportional to the ratio of thicknesses. With this change of velocity, the chips are caused to travel in the direction of a thickness dimension over varying distances, while travelling in longitudinal or lateral direction at the same velocity over the same distance.
Alternatively, this type of apparatus can be understood to sort wood chips according to their surface to weight ratio. Wood chips having the same length and width, but different thicknesses will have different surface to weight ratios. Thicker chips will have a smaller surface to weight ratio than thin chips. The change in velocity and direction resulting from exposure to an impulse action of gas pressure for a thick chip with a small surface to weight ratio will be less than the change of velocity and direction of a thinner chip with a greater surface to weight ratio exposed to the same impulse action. This type of sorting is most effective when the largest area surface of the chips are exposed to the impulse action.
The application of an impulse action or effect on a desired chip surface requires that the chips be directed to have their largest area surface facing perpendicularly the application direction of a dynamic pressure. The establishment of a given application time requires that the chips travel at the same speed across the pressure application site of a given size.
The chips slide along a sloping plane over a given distance for a time sufficient to set themselves in a proper position and to attain a given velocity. At this speed, the chips slide across a narrow slit, the air flowing therefrom applying its dynamic pressure to provide an impulse effect or action. The chips fly freely in the airspace, thus having a common speed component downwards and, consistent with the above calculation, a thickness-specific speed in horizontal direction, whereby the chips of non-uniform thicknesses fly to various distances from the site of impulse action: thinner chips fly farther away.
High-density scrap particles (e.g. steel has a density which is about tenfold in comparison to solid density of wood) have just a slight change of speed, and the same applies to irregularly shaped particles, due to aerodynamic properties. This way, such particles can be separated from a flow of chips in a single process. Respectively, fine dust particles obtain a major change of speed, due to a thickness dimension being very small. Consequently, dust separates effectively to form a separate flow.
Various fractions are collected from the separated flow of chips by setting up partitions or separating walls in appropriate places. The partitions can be adjustable for varying the chips content of fractions, as necessary. This also enables the implementation of a resorting process for a desired fraction by the same method.
In comparison to other equipment known from the prior art, this type of apparatus requires less maintenance, the number of moving parts being radically reduced. Mechanical strength is also improved throughout the apparatus by virtue of a minor fatigue stress, especially the fact that no reciprocating or gyroscopic motion is necessary. In terms of its adjustability, the apparatus can be designed to be easier and quicker than those available at present.
The support surface for sliding wood chips along for sorting the same in a variety of fractions must be set at a relatively large angle relative to the horizontal plane. Thus, an air cushion develops easily between individual chips and the support surface, having an adverse effect on the apparatus in terms of its operation, since the strength of impulse action upon a single chip thus fluctuates extensively, depending on the air cushion thickness. Therefore, the apparatus does not have a best possible sorting capacity.